It is well-known in the technical field above that the sound quality of telephones is generally not always perfect. The main reason for this is the limited frequency response (between approximately 300 and 3400 Hz) of the telephone. The limits for the frequency response are specified in various approval regulations, which are mostly based on recommendations from the no longer existing CCITT (the International Telegraph and Telephone Consultative Committee), which used to be a committee of the ITU (International Telecommunications Union), which in turn is a United Nations treaty organization. In order to ensure repeatable measurements the recommended test methods make use of well-defined equipment, such as an artificial ear and mouth used for frequency response measurements.
According to a generally accepted test approach the frequency response of the loudspeaker in a telephone is measured by means of a test microphone coupled with an airtight seal directly over the loudspeaker. The test microphone is normally attached at the center of an earpiece portion of the telephone housing, i.e. immediately above the loudspeaker, which is located behind said earpiece portion. It is well within reach for manufacturers of state-of-the-art telephones to make the frequency response sufficiently flat and within the prescribed limits during testings with aforesaid air-tight seal between the loudspeaker and the microphone, so that the telephone may pass the tests.
However, since the tests prescribe an air-tight seal as described above, the frequency response of the loudspeaker will be less satisfactory, if even a very small gap exists between the loudspeaker and the microphone. In real life there is rarely a perfect seal between the loudspeaker and the ear of the user; on the contrary, the telephone will normally be kept at a small distance from the human ear. Particularly as regards mobile telephones, where the receiving loudness may be high and the level of noise may be considerable, the user will most probably put some distance between his ear and the loudspeaker of the telephone. The consequence of this is that the lower frequency region of the sound received through the loudspeaker will not exhibit the perfect loudness shown during test approvals; instead the sound amplitude at low frequencies will be considerably lower than for an ideal situation with a flat frequency response.
Previously known remedies for the problem above have involved boosting the low-frequency region of the sound emitted from the loudspeaker, wherein difficulties may arise in maintaining the overall frequency response of the telephone at a sufficiently flat level. Additionally, one obvious approach would be to provide an overall reduction in noise by using acoustical, electrical and electronic components of premium quality. However, this approach will imply a cost penalty to the total price for manufacturing the telephone.